Concentrated aqueous H.sub.2 SO.sub.4 solutions may be obtained by evaporating diluted solutions of any origin and of any concentration.
In general, the solutions most commonly available are those which come from the processes for the production of titanium dioxide via sulphate or from the pickling treatments of metals.
Such solutions usually contain from 5 to 25% by weight of H.sub.2 SO.sub.4, as well as various impurities such as Mg and possibly Fe, Ca, Al, Cr, V and Mn.
For instance, the solutions coming from the production of titanium dioxide via sulphate, in general have the following composition by weight:
H.sub.2 so.sub.4 : 15-23% PA1 Mg: 0.07-0.4% PA1 Fe: 1-55 PA1 TiO.sub.2 : 0.06-0.6%
and in that solution there are present other impurities such for instance: Ca, Mn, Al, V and Cr.
When diluted aqueous solutions of H.sub.2 SO.sub.4, containing Mg, are concentrated, for instance up to above 65% by weight, using any known method (for instance by evaporation concentration under vacuum or in submerged flame), it turns out that the precipitation of the Mg is very limited in time. The same phenomenon can be observed, even if on a lower extent, when there are present Al, Fe, V, Cr or Mn.
More particularly, when filtering slurries containing for instance 65-75% by weight of H.sub.2 SO.sub.4 and 0.1-0.5% by weight of Mg, one obtains a filtrate which at the end of the filtering operation will appear clear but which subsequently will become roiled, often even acquiring a gelatinous consistency. In fact, in the filtrate there continue to precipitate for various days solids that prevailingly consist of magnesium sulphates.
The thus precipitated solids are in the form of crystals with a maximum size not exceeding 10 microns, that is they are extremely fine and bulky, wherefore a possible filtering would prove difficult and would produce cakes with a very high imbibition.
On the other hand, the concentrated sulphuric acid is practically unusable if one does not separate the solid present in the suspension.